After years of delays, the U.S. Food and Drug Administration (FDA) has introduced a law requiring restaurants and other food outlets with 20 or more locations to post calorie counts.

The rule, formally named 'Food Labeling; Nutrition Labeling of Standard Menu Items in Restaurants and Similar Retail Food Establishments', had an original compliance date of 2015. That was extended three times to help the industry understand and prepare for the rules.

In anticipation of the law, big firms like McDonald's and Starbucks have already introduced the calorie information on their menus and menu boards.

One example is a "Big Mac Meal" with regular fries and a full sugar coke which McDonald's now openly states as having 1,120 calories.

According to the 2015-2020 Dietary Guidelines for Americans, women are likely to need between 1,600 and 2,400 calories a day, and men from 2,000 to 3,000.

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Customers order food from a McDonald's restaurant in Des Plaines, Illinois.

The new rules are a President Obama-era innovation and were put into legislation as part of the Affordable Care Act in 2010.

The Commissioner of the U.S. Food and Drug Administration, Scott Gottlieb, said in a blog post Wednesday that information about how healthy food gives people the chance to "make better choices about our diets."

Gottlieb also claimed in his blog post that Americans drink or eat about one-third of their daily calories outside the home but labeling can reduce that number.

It is estimated by the National Center for Health Statistics that 40 percent of all Americans are obese. Doctors say obesity increases the chances of developing heart disease, diabetes, and some types of cancer.